I have tried Mendeley, but despite its odd (Swig-based) interface, I prefer JabRef (http://jabref.sourceforge.net/). It is open-source, and it keeps its database of references in plain BibTeX format: therefore, it is super-easy to edit it by hand, when necessary. It does not offer facilities for searching/discovering new papers, but IMO it is perfect to manage a database of citations.

I abandoned Mendeley back when I realised I could not use it whilst keeping my reference collection synced via Dropbox.

That led me to try Zotero, and I have not looked back. Though I wish the Zotero devs would just hurry up and give us an editable bibtex citation key field!

On OS X, I like Papers (http://www.papersapp.com/) since it allows me to download the papers directly from the application.

pops up every so often, don't forget Qippa (http://www.qiqqa.com/)

*i'm not affiliated with qippa, it's just what i prefer over mendeley